Dynamic response of plasma energy and broad-band magnetic fluctuations to additional heating in JET

The discrepancy between the transport coefficient inferred from analyses of the steady-state power balance and of the evolution of temperature profile perturbations, and the observation that the heat flux may not be simply proportional to the local product n∇T, have led to conclude that the energy confinement in JET is determined either by an effective thermal conductivity which is itself a function of ∇T, or by an energy balance containing a term formally equivalent to a 'heat pinch'. The former interpretation was less favoured by comparison with the results of heat pulse propagation studies, but could not be definitely excluded. The latter has found its most successful embodiment to date in the 'critical temperature gradient' model by Rebut and Lallia. From the point of view of global parameters in steady-state, a similar picture was inferred from the observed relationship between energy content W and input power P, leading either to a Goldstone-like power law (W∼Wo+τEincP) representation for the energy content. Both scalings provide good fits to the JET data. Within the framework of an offset linear model, while the energy replacement time τE degrades with input power, the actual plasma transport properties are best characterized by a constant 'incremental' time τEinc=dW/dP. τEinc will reflect the form of the local anomalous thermal conductivity induced by ergodization of the magnetic field lines. In this paper we show that τEinc can be considered as an effective timescale for the global energy time evolution in response to variations in the input power. The time evolution of the edge measured magnetic fluctuations in the same discharges has been modelled with another timescale, different in principle from the energy one, that has been found very well correlated with it. (author) 5 refs., 4 figs